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Summary of Performance option activity (Detail) - Performance Based Stock Options
3 Months Ended
Mar. 31, 2019
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Incentive Stock Option Plans, Beginning balance | shares 140,411
Incentive Stock Option Plans, Option exercised | shares (5,735)
Incentive Stock Option Plans, Ending balance | shares 134,676
Weighted Average Price Per Share, Beginning balance $ 11.49
Weighted Average Price Per Share, Options exercised 11.49
Weighted Average Price Per Share, Ending balance 11.49
Execisable Weighted Average Price Per Share, Beginning balance 11.49
Execisable Weighted Average Price Per Share, Options exercised 11.49
Execisable Weighted Average Price Per Share, Ending balance $ 11.49